Powder Coating should be more durable. The difference between Powder Coat and Paint is just the application method. Paint is sprayed on in a liquid form and chemically bonds to the surface, Powder Coat is actually applied as the name states, in a powder form and then baked on. In the oven, the baking causes the powder to melt/flow together and bond to the surface. There is also ESP or electrostatic painting where there is a positive negative charge through the paint and surface to promote a bond.

Ultimately, the bonds are similar after the pigments are set up but Powder should be more resistant to abrasions to some degree. Having said all that, if you never plan to actually load much on the racks, you could probably sand them, prime them with a nice marine grade primer and top coat them and be just fine.